How to make toothbrush Bristle animation
As suggested by mma78 in the comment try a Hair Particle System.

Brushing the default cube

Make an emitter plane:

Copy a few faces of the lower side of the toothbrush head where the brushes sit. In Edit mode, duplicate Shift+D, press Esc or RMB to leave it where it is, and separate selection with P)
In Object mode, rename the plane, and parent it to the toothbrush (Ctrl+P). Parent must be selected last = highlighted light orange.


Add a Hair Particle System to the bristle plane

150 particles
3, 5, or 8 segments (I used 8. Test it, less can make the bristle stiffer)


In Field Weights turn off gravity:

Gravity: 0.000  (we do not want to bend the bristles to the ground)


Adjust these values for render and the shape/appearance of the bristles:

Render:

[_] Show Emitter (disabled, we don't want to render it)
Path → Steps: 5


Viewport Display:

Strand Steps: 5


Children: Simple

Radius: 0.02m (adjust this to your toothbrush size! Value here is for the 2m default cube!)
Roundness: 1.000


Clumping:

[X] Use Clump Curve
(edit a curve. Make it round in the top right corner. This makes the bristles heads round)


Hair Shape:

Strand Shape: -0.850    (for rounded bristles)
Diamenter Root:  0.3m    (adjust this to your toothbrush size!)




enable [X] Hair Dynamics. Run/bake the simulation and play with the Hair Dynamics values:

Collision

Distance: 5cm (adjust this to your toothbrush size!)


Pin Goal Strength: 10
Structure:

Vertex Mass: 0.3 kg (default, higher values make the bristles inert)
Stiffness: 30 (higher values result in less bending)
Damping: 0.300 (default, increase if the bristles swing too much)





Happy teeth brushing!